http://www.nongnu.org/rdiff-backup/rdiff-backup.1.html
--preserve-numerical-ids
If set, rdiff-backup will preserve uids/gids instead of
trying
to preserve unames and gnames. See the USERS AND GROUPS
section
for more information.
I would also read the USERS AND GROUPS section for the caveats and other
ways to deal with this.

Better test that first. I believe that the original Uname is still
recorded in the mirror_metadata files, and any difference would
be treated as a change. Mapping of UIDs vs. Unames would affect
the actual ownership of the files in the mirror and increments
directories and the ownership of restored files.
--
Bob Nichols "NOSPAM" is really part of my email address.
Do NOT delete it.